In some communication networks it is common practice that network nodes are equipped with two or more wireless transceivers in order to facilitate communication over different electromagnetic channels. In these cases, interferences are very likely to occur between the different channels due to the physical vicinity of a node's different transmitters. This interference problem gets worse when the transceivers operate in electromagnetic channels that are rather close or even adjacent.
A scenario where the problems outlined above might occur is in vehicular ad hoc networks. Vehicular communication is considered a key technology for Intelligent Transport Systems (ITS) because it has the potential to increase road safety and traffic efficiency. For this purpose, the mature, inexpensive, and widely available 802.11 wireless LAN technology appears very attractive. In vehicular communication, vehicles are equipped with wireless transceivers and can spontaneously form an ad hoc network among them. Vehicles acting as network nodes can use the ad hoc network to communicate with each other in order to support safety applications such as cooperative collision warning.
Recognizing the potential of vehicular communication, the European Commission has recently allocated a 30-MHz frequency band (5875-5905 MHz) for safety-related communication of Intelligent Transport Systems (Commission Decision of Aug. 5, 2008 on the harmonized use of radio spectrum in the 5 875-5905 MHz frequency band for safety-related applications of ITS, 2008/671/EC). While the European Commission has not specified how this frequency band will be used, based on the current status of standardization activities in ETSI TC ITS (European Technical Standards Institute Technical Committee Intelligent Transport Systems), it is expected that this frequency band will be divided into one 10-MHz control channel (CCH) and two 10-MHz service channels (SCH1 and SCH2). An overview of the anticipated European channel allocation for safety-related communication is depicted in FIG. 1. Additional 10-MHz service channels in the same 5.8-5.9 GHz frequency band are expected to become available in Europe for other vehicular applications.
Further, as the automotive industry requires that a vehicle is constantly able to receive messages sent on the control channel, the most appropriate solution according to currently and short/mid-term available hardware consists of a dual transceiver communication system including two transceivers operating in this frequency band. One of these transceivers will operate in the control channel that is dedicated to the exchange of periodic messages and event-driven warning messages. The other transceiver will operate alternately on the service channels and will be used for other ITS-related communication purposes.
As can be observed in FIG. 1, CCH, SCH1, and SCH2 do not overlap. Thus, in theory, these channels are orthogonal, i.e., signals transmitted simultaneously on these channels do not interfere with each other. However, interference between these channels occurs because they are adjacent (SCH2-CCH, SCH1-SCH2) or near (SCH1-CCH). Interference occurs when, due to imperfect transmit spectrum mask, a signal transmitted on a channel can spill over into adjacent channels and interferes with signals on these channels. Other causes of interference are limitations of receivers' filters and problems due to the co-location of two transceivers in the same unit, like board crosstalk and radiation leakage. The consequences of interference include: (i) on the transmitter's side, adjacent and near channel interference induces spurious carrier sense for CSMA (Carrier Sense Multiple Access)-based wireless communication technologies; (ii) on the receiver's side, adjacent and near channel interference creates a variant of the hidden node problem since signals spill over into adjacent channels and impair each other's reception.
Several measurement studies indeed confirmed the problem of adjacent and near channel interference. In this respect it is exemplarily referred to Nachtigall et al., “The Impact of Adjacent Channel Interference in Multi-Radio Systems using IEEE 802.11”, Proceedings of the International Wireless Communications and Mobile Computing Conference, August 2008 as well as to Angelakis et al., “Adjacent Channel Interference in 802.11a: Modeling and Testbed validation”, IEEE Radio and Wireless Symposium (RWS 2008), Orlando, Fla., USA, January 2008. The studies have identified different interference scenarios in 802.11 networks, which are conveniently classified according to the source that generates interfering signals. The scenarios addressed by the present invention are those where simultaneous activities on multiple transceivers take place within the same note (i.e. “locally” interfering). In the TX-TX scenario, two simultaneous transmissions interfere with each other and thus limit the probability of correct reception on the receiver's end. In the TX-RX scenario, an ongoing reception is disturbed by an initiating transmission.
The vehicular draft version of 802.11 (IEEE P802.11p/D6.0—Draft Standard for Information Technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications—Amendment 7: Wireless Access in Vehicular Environments) recognizes the problem of adjacent channel interference and introduces 4 classes of transceiver's spectrum mask, where the highest class D outperforms currently available hardware. The additional complexity required to achieve the class D spectrum mask is rather high and strongly impacts the final per-unit transceiver cost, representing a major concern for the automotive industry and therefore reducing the likelihood that class D transceivers will ever be deployed.
Various techniques have been proposed in other short-range wireless applications like mesh networks in order to limit the effects of adjacent channel interference, including the usage of directional antennas (e.g. in Cheng et al., “Adjacent Channel Interference in Dual-radio 802.11 Nodes and Its Impact on Multi-hop Networking”, IEEE Global Telecommunications Conference (GLOBECOM 2006), November 2006) and the selective usage of channels with enough distance in frequency between them (e.g. in V. Raman, “Dealing with Adjacent Channel Interference Effects in Multichannel, Multi-interface Wireless Networks”, Master's Thesis, University of Illinois at Urbana-Champaign, December 2008.). However, these techniques are not suitable for various scenarios and are in particular not applicable to vehicular communications where network nodes are not stationary and the only available channels are adjacent or near.
A related technique for improving the throughput of wireless access points equipped with multiple radios operating on different channels is described in US 2005/0286446 A1. The technique disclosed addresses a pathological scenario where a wireless access point first sends a unicast data frame to a client on a certain channel and then transmits a unicast data frame to another client on another channel shortly thereafter. In this case, the first client sends an acknowledgement to the wireless access point while the second transmission is still ongoing. Due to channel interference, the first client's acknowledgement cannot be received correctly by the wireless access point. This causes the wireless access point to do one or multiple retransmissions and delivers poor performance. The proposed technique solves this pathological scenario by synchronizing all radios of a wireless access point so that its transmissions on multiple channels begin and end at approximately the same time. This technique is not applicable for vehicular communication that operates in ad hoc mode and does not have an access point. Further, vehicular communication transmits safety-related messages in broadcast frames that do not have an acknowledgement mechanism.
US 2003/0147368 A1 describes another related method according to which the multiple transmitters co-located in the same station are alternatively toggled on/off. Switching on and off a transmitter implies that the transmitter being turned off will eventually drop the packets that are accumulating in its transmit queue. Furthermore, toggling the status of a transmitter introduces latency due to the time required by a transmitter to return to its active state.
The solution described in US 2008/0232339 A1 introduces a cross-technology activity check but only reacts to possibly interfering transmissions by rejecting one and counting the number of rejections as an indication based on which the activity of the other transmitter is reduced. This statistically allows both transmitters to have the chance to transmit without causing interference to each other, but does not enable packet re-scheduling with the minimum possible latency that still allows preventing interfering transmissions. The simple rejection implies high latencies and the statistical interference mitigation requires the reduction of activity on one transmitter in order to be effective. In particular for safety applications these performance characteristics are not acceptable.
It is therefore an object of the present invention to improve and further develop a method for coordination of wireless transceivers of a network node and a network node of the initially described type in such a way that, by employing mechanisms that are readily to implement, interferences caused by simultaneous transmissions of different transceivers are widely eliminated in a reliable and efficient way.